Method for cleaning an oil field capillary tube

ABSTRACT

A method and apparatus for removing deposits formed within a capillary tube that has been used to deliver treatment chemicals into a well. The method includes pumping a cleaning solution through the capillary tube coil, preferably after the capillary tube has been removed from the well and formed as a coil. The cleaning solution comprises at least 20 weight percent of a surfactant or dispersant, at least 5 weight percent of a coupling agent, and at least 10 weight percent solvent. A preferred surfactant or dispersant is selected from the group consisting of an alkyl-aryl sulphonate and a phosphate ester. An example of a cleaning solution includes dodecylbenzeneylsulphonic (DDBSA) acid, ethylene glycol monobutyl ether and toluene.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent application61/037,733, filed on Mar. 19, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods for cleaning and maintaining anoil field capillary tube.

2. Description of the Related Art

Capillary tubes are used in oil field applications to inject varioustreatment chemicals into a production well. A capillary tube typicallyhas a diameter from about one eighth (0.125) inch to about 2 inches andmay extend several thousand feet down into a wellbore. Treatmentchemicals are pumped into one end of the capillary tube at the surfaceand exit the other end of the capillary tube situated deep in thewellbore, or at a subsea tree in the case of an offshore subsea design.Accordingly, the treatment chemicals are allowed to contact theproduction fluids, wellbore casing, and other equipment associated withthe production stream.

The treatment chemicals selected for pumping into the wellbore maydiffer from well to well depending upon the properties of the fluid(s)being produced. For example, if the production fluid is corrosive to thewell tubing and also tends to form scale deposits, then the treatmentfluid may include both a corrosion inhibitor and a scale inhibitor.Other properties of a production fluid may lead to the use of yet othertreatment chemicals, as is generally known in the art. The one or moretreatment chemicals pumped into the wellbore are selected to maintain oroptimize the profitability of the well. Generally, the profitability ofthe producing well is a function of the rate of production, the cost oftreatment chemicals and other well operating costs.

Therefore, the ongoing addition of treatment chemicals through thecapillary tube is important to the continued operation of the well.However, capillary tubes are prone to plugging. Stoppage and/orreduction of capillary flow can be in the form of a localized plug ofsolid or semi-solid material, a layer of material (thus reducing theeffective internal diameter of the capillary tube), or both. Formationof the plugging/flow-reducing build-up can stem from many causes. Whenthe desired treatment chemicals are no longer added to the wellbore,production rates can drop significantly or even come to a halt and causea substantial loss of revenue. Also the applications of certainproduction chemicals, such as corrosion inhibitors, are needed to reducethe risk to the asset which can be costly over the producing life of thewell.

Conventional techniques for unplugging a capillary tube include removingthe capillary tube from the well and pumping water through the capillarytube at high pressure until the plug is overcome and water begins toflow from the other end. The unplugged capillary tube may then be placedback into service within the well.

However, the present inventors have observed that a capillary tube thathas been recently unplugged may develop a new plug rather quickly.Investigating the problem further, the inventors identified that theconventional technique for unplugging a capillary tube may serve toremove a localized plug, but does little or no cleaning of the innerwall of the capillary tube. As a result, simply unplugging a capillarytube using a high pressure fluid was found to leave solid deposits overlarge amounts of surface area within the capillary tube, and thesedeposits could quickly cause the capillary tube to again become plugged.

Therefore, the present inventors identified a need for a method ofcleaning a capillary tube. It would be desirable if the method removedsolid deposits from throughout the capillary tubing so that thecapillary string performed similarly to a new capillary tube. Inparticular, it would be desirable that the method removed sufficientamounts of solid deposits so that the capillary string would not becomeplugged more rapidly than a new capillary tube.

SUMMARY OF THE INVENTION

One embodiment of the invention provides a method and apparatus forremoving deposits formed within a capillary tube that has been used todeliver treatment chemicals into a well. The method includes pumping acleaning solution through the capillary tube coil (after the capillarytube has been removed from the well and is directed to form a coil forcleaning purposes). The cleaning solution comprises at least 20 weightpercent of a surfactant or dispersant, at least 5 weight percent of acoupling agent, and at least 10 weight percent solvent. A preferredsurfactant or dispersant is selected from the group consisting of analkyl-aryl sulphonate and a phosphate ester. Dodecylbenzenesulphonicacid is particularly preferred. Depending on the pH of the cleaningformulation, surfactants having ionizable moieties may be in hydrogenform, or salt form. If in salt form, the counter cation will correspondto the conjugate base, or mixture of conjugate base species that werepresent before blending the components. pH can be controlled in all suchformulations described in this invention, either by adding a weak acid,weak base, strong acid, or strong base in amount sufficient to maintainformulation stability, that is, to keep all components in solution, andto maintain proper pH for optimum capillary string cleaning and minimalmetal loss, if any, from the capillary string. A specific embodiment ofthe cleaning solution includes dodecylbenzeneylsulphonic (DDBSA) acid asthe surfactant or dispersant, ethylene glycol monobutyl ether as thecoupling agent and toluene as the solvent.

Generally embodied in this invention is the use of any type ofsurface-active agent (surfactant) or admixture of surfactants combinedwith a suitable solvent system. Said solvent system could be aqueousand/or organic based. These surfactants can be anionic, cationic,nonionic, zwitterionic, or amphoteric. Any other compound having surfaceactive properties, such as hydrotropes, would also be within the scopeof this invention. Hydrophobic moieties of said surfactants can belinear, branched, contain unsaturated regio-chemistry, and have regionsof polar atom substitution to the extent that the chain can still beconsidered hydrophobic. Furthermore, the action of the cleaning mixture,while being described as a surfactant, dispersant, or wetting agent, canbe described in more general terms as an emulsifier, solubilizer,detergent, or anti-redeposition agent.

Another embodiment of the invention provides a method for removingdeposits formed within a capillary tube that has been used to delivertreatment chemicals into a well, comprising flowing a cleaning solutionthrough the capillary tube and into the well, wherein the cleaningsolution comprises at least 20 weight percent of a surfactant ordispersant, at least 5 weight percent of a coupling agent, and at least10 weight percent of a solvent. Optionally, hydrocarbons may be producedfrom the well while the cleaning solution flows through the capillarytube.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view of a capillary tube coil secured on aspool.

FIG. 2 is a schematic diagram of a system for cleaning a capillary tubecoil.

DETAILED DESCRIPTION

The present inventors have discovered that the solids that restrict andplug a capillary tube are formed from combinations of well treatmentfluids that were previously believed to be stable under a full range ofoperating conditions. Now it has been discovered that well treatmentfluids may experience conditions in which the combinations of welltreatment chemicals interact to form solids that adhere to the insidesurface of a capillary tube. The conditions that give rise to theformation of solids in a capillary tube may not be uniformly experiencedin all fields, in all wells within a field, or at all positions within asingle capillary tube. For example, the hardness of the source of waterused to blend with the treatment chemicals may affect solids formation.Furthermore, the temperature of the well treatment fluids risesconsiderably with the increasing depth of the capillary tube and thedepth of the wells can vary significantly. Still further, variationsbetween producing formations may lead to the use of differentcombinations of the treatment chemicals from one well to another.Accordingly, the properties of the treatment chemicals, the exact ratiosof the chemicals, and the range of conditions that may be experienced ina capillary tube can cause the deposition of solids. The exact nature ofthe solid may be crystalline, semi-crystalline, amorphous, or liquidcrystalline (lamellar, cubic, etc.). The solid may also be in the formof an aggregate, having dispersed crystalline particles in an amorphousor liquid crystalline matrix.

One embodiment of the invention provides a method of cleaning acapillary tube that has developed solid deposits that restrict or plugthe flow of treatment chemicals into a well. Optionally, the method maybe performed by pumping a cleaning solution through the capillary tubein situ and into the well so that the capillary tube does not have to beremoved. If the well can continue to produce without treatment chemicalsduring the period of the in situ cleaning, then hydrocarbons maycontinue to be produced from the well during the performance of thecleaning method. However, the method is preferably performed by pumpinga cleaning solution through the capillary tube after the capillary tubehas been removed from the well. The capillary tube is typically coiledonto a spool or other device for compact and organized handling of thecapillary tube. Having removed the capillary tube from the well, thescope and duration of the cleaning method are not constrained by thewell conditions. If desired, a different capillary string may be runinto the well to continue the supply of treatment chemicals.

FIG. 1 is a perspective view of a capillary tube coil 10 secured on aspool 12. The spool 12 has two circular ends 14 that can be used tohandle the spool and protect the capillary tube coil. A generallycylindrical body 18 (shown by dashed lines) extends between the circularends 14 about a central axis 16. The diameter of the cylindrical body 18is about one foot so that the initial layers of capillary tube coiledonto the spool are not bent or kinked. A hole 20 is made through acircular end 14 radially adjacent the outer surface of the cylindricalbody 18 to receive a first end of the capillary tube prior to coiling.The first end 22 of the tube 10 is secured through the hole, such asusing large staples. The capillary tube is then coiled about the body 18such that successive layers of capillary tube are coiled about theinitial layers up to a maximum diameter of about four feet. A second end24 of the capillary tube 10 is secured to the spool to prevent the tubefrom accidentally coming uncoiled during handling.

Once a capillary tube has been removed from the well for cleaning, thecapillary tube may need to be conditioned for receiving a cleaningsolution. For example, where the capillary tube has been in servicedelivering aqueous treatment fluids to the well, as is typically thecase, and the cleaning solution would cause the formation of a stableemulsion, then a substantial amount of the water remaining in thecapillary tube should be removed. The water may be removed from thecapillary tube by passing an alcohol through the tube. Although variousalcohols may be used, isopropyl alcohol is particularly preferred. It isgenerally not necessary to completely fill the entire volume of thecapillary tube or pass multiple volumes through the capillary tube.Rather, the water may be sufficiently removed by pumping a small plug,such as 10 percent of the capillary tube volume, of an alcohol into thetube ahead of the cleaning solution.

In accordance with the invention, a cleaning solution is pumped throughthe capillary tube. Preferably, the method includes at least one periodof pumping the cleaning solution through the capillary tube at avelocity of between 0.1 and 2.5 meters per second, which may beaccomplished by controlling the volumetric flow rate. The high velocityof the cleaning solution and the interactions at the interface betweenthe cleaning solution and deposits within the tube are an important partof the cleaning process.

One embodiment of the invention provides a cleaning solution thatcomprises at least 20 weight percent of a surfactant or dispersant.Preferably, the cleaning solution comprises between 20 and 70 weightpercent of the surfactant or dispersant. A particularly suitablesurfactant or dispersant is selected from the group consisting of alkylaryl sulphonates and phosphate esters. These surfactants can be anionic,cationic, nonionic, zwitterionic, or amphoteric. A preferred alkyl-arylsulphonate is dodecylbenzenesulphonic acid (DDBSA).

Optionally, the cleaning solution may further comprise at least 5 weightpercent of a coupling agent. Preferably, the cleaning solution comprisesbetween 5 and 30 weight percent of the coupling agent. A preferredcoupling agent is ethylene glycol monobutyl ether. Examples of othertypes of glycol ether coupling agent include, but are not limited to,short-chain alkyl ethers of ethylene glycol, propylene glycol, orbutylene glycol, phenolic ethers of ethylene glycol, propylene glycol,or butylene glycol, and the like. Acetates, ethers, and otherderivatives of these coupling agents could also be employed, providedthat they impart coupling action, such as a low molecular weight alcoholcosolvent.

Still further, the cleaning solution may comprise at least 10 weightpercent solvent, such as toluene. Preferably, the cleaning solutioncomprises between 10 and 20 weight percent of the solvent. Otheraromatic solvents like xylenes and others are useful. Branched, cyclic,linear, unsaturated hydrocarbons, and the mixtures thereof, can also beused in this application. Aliphatic solvents alone or in combinationwith aromatic solvents may also be used. These solvents may also includeolefinic functional groups.

Cleaning formulations within the scope of this disclosure can becomposed of any type of surface-active agent (surfactant) or admixtureof surfactants combined with a suitable solvent system. Said solventsystem could be aqueous and/or organic based. These surfactants can beanionic, cationic, nonionic, zwitterionic, or amphoteric. Any othercompound having surface active properties, such as hydrotropes, wouldalso be within the scope of this patent. Furthermore, the action of thecleaning mixture, while being described as a surfactant, dispersant, orwetting agent, can be described in more detail as an emulsifier,solubilizer, detergent, or have anti-redeposition properties. Stillfurther, the initial mode of action for the cleaning operation on amicro-scale, is the formation of a surfactant interface on the pluggingmaterial. This interface is formed by surfactant molecules thatspontaneously align and form a single mono-layer, or multi-layeredstructure on the plugging material. Solvent molecules can also beinvolved in helping to break up the plugging material in the presence ofsurfactants. The dispersed plugging material is then solubilized and/ordispersed by various mechanisms, such as roll-up, micellarsolubilization, and the like. Surfactant aggregates, such as micelles,can act to maintain solubility of the removed plugging material byinternal sequestration inside the structure. It is anticipated that,depending on the polarity of the liquid medium used, that reversedmicelles or normal micelles will be in preponderance, thus acting tosolubilize polar and non-polar plugging components as needed. Surfactantaggregates may consist of spherical micelles, worm-like micelles, orliquid-crystalline aggregates that are sufficiently mobile to remove theplugging material from the capillary tubing. Furthermore, the internalstructure of these surfactant aggregates can act as a solubilizationlocus for the solvent component and coupling agent of the cleaningsystem to a given extent. It is further known that a certain fraction ofthe solvent, surfactant, and coupling agent alike will have a definitebaseline free solubility in the aqueous medium. Moreover, there will bean equilibrium between free solvent and coupling agent in solution andthat found solubilized in surfactant aggregates.

The duration of pumping the cleaning solution through the capillary tubecoil may be variable or fixed. For example, the cleaning solution may bepumping through the tube for a fixed period of at least two hours.However, another embodiment includes the steps of measuring the pressuredrop and flow rate of the cleaning solution pumped through the capillarytube coil, and continuing to pump the cleaning solution through thecapillary tube coil until the capillary tube coil is determined to beclean. For example, the capillary tube coil may be determined to beclean if the measured pressure drop and flow rate are within 15 percentof a pressure drop and flow rate measured on a new capillary tube coilhaving the same nominal length and diameter under the same nominalpressure and other conditions. Other embodiments may determine the tubeto be clean on the basis of a total volume of solids removed/filteredout of the cleaning solution or a reduction in the rate of solidsremoval/filtration.

A further embodiment comprises pumping the cleaning solution into thecapillary tube string at a constant supply pressure, measuring a flowrate of the cleaning solution being pumped through the capillary tubecoil, and continuing to pump the cleaning solution through the capillarytube coil until the measured flow rate is within 15 percent of a flowrate that would be expected through a new capillary tube coil having thesame nominal length and diameter under the same nominal pressure andother conditions.

Similarly, a separate embodiment comprises pumping the cleaning solutionthrough the capillary tube coil at a constant flow rate, measuring thepressure drop of the cleaning solution between the ends of the capillarytube coil, and continuing to pump the cleaning solution through thecapillary tube coil until the measured pressure drop is within 15percent of a pressure drop that would be expected through a newcapillary tube coil having the same nominal length and diameter underthe same pressure and other conditions.

In yet other embodiments, the step of pumping the cleaning solutionthrough the capillary tube may be simultaneously or sequentiallycombined with an additional step or process selected from (1) reversingthe flow direction of cleaning solution being pumped through thecapillary tube coil, (2) changing the orientation of the capillary tubecoil, (3) heating the capillary tube coil or the cleaning solution sothat the capillary tube coil or the cleaning solution have a temperaturegreater than ambient temperature, and combinations thereof. Stillfurther, the cleaning solution may remain in the capillary tube coil(i.e., soaking) for a period greater than one hour without pumping. Itshould also be recognized that one or more of the foregoing steps may berepeated any number of times to achieve the desired result of cleaningthe capillary tube. During these operations of flowing, soaking, reverseflow, pulsed flow, or heating, the cleaning mixture will be effectivelywetting, emulsifying, dispersing, and solubilizing components of theplugging material. Two main processes come into play in these scenarios.One being the kinetics of removal, and the other being energy input tophysically aid in removal of sequestered/solubilized plugging material.

The capillary tube may be heated by thermal conduction, convection,radiation, or a combination thereof. Furthermore, the method may includethe direct or indirect heating of the tube or the cleaning solution. Forexample, an entire capillary tube coil may be placed inside a heatedchamber, which facilitates the heating of the cleaning solution while itis flowing through the coil.

The deposits within the capillary tube may be soaked with the cleaningsolution by pumping the cleaning solution into the tube, then shuttingoff the pumps. Accordingly, the time duration of contact between thecleaning solution and the deposits may be increased without the expenseof continued pumping.

Optionally, the direction in which the cleaning solution is flowingthrough the tube may be reversed in order to introduce a new dynamic tothe cleaning mechanisms operating within the tube. The fluid experiencesalternate locations of turbulence in reverse flow which increases thefrictional interactions along the surface area between the solution andthe solids.

In a further option, ultrasonic waves may be induced on the capillarytube so that the mechanical energy of creating and collapsingmicroscopic cavitation bubbles within the fluid inside the string willhelp break up particle adhesions. Other methods for causing vibration ofthe deposits may also be utilized.

In a still further option, the capillary tube coil may be repositionedand/or rolled about one or more axis of rotation to vary the fluidtrajectory within the capillary tube and assist with particle removal.Repositioning and/or rolling may also increase the removal of particlesby overcoming the force of gravitational, frictional, and velocitylimiting effects.

Cleaning the capillary tube while the tubing is coiled around a spoolcomplicates the cleaning process by dramatically increasing the pressuredrop through the tube from about 100 psi for a linear capillary tube toperhaps 6,000 psi for the same capillary tube disposed in a coil ofabout 1-4 feet in diameter. The absence of a reliable flow model forcoiled tube having such a small diameter makes it difficult to determineif the capillary tube is clean. Accordingly, certain embodiments of theinvention utilize a collection or database of fluid pressure and fluidflow measurements as a function of diameter and length for new (clean)coiled capillary tubes. Used capillary tubes are preferably cleaneduntil the pressure and flow measurements meet established criteria, suchas a setpoint percentage or standard deviation, relative to thebenchmark values for a new capillary tube of substantially the samelength and diameter under substantially similar conditions.

One embodiment of the invention uses measurements of the cleaningsolution pressure and/or flow rate through the capillary tube as aquantitative indicator of the degree of cleanliness for an in-service orout-of-service capillary tube. A database of pressure and/or flow ratemeasurements are collected, wherein each database record includes, forexample, the pressure, flow rate, temperature, fluid composition,capillary tube diameter, capillary tube length, and pump specifications.An application program utilizes pressure and/or flow rate measurementsfrom a given capillary tube to calculate/estimate the cleanliness of thegiven tube based upon a comparison against data previously collectedfrom capillary tube cleaning runs involving identical parameters, suchas pressure, flow rate, temperature, fluid composition, capillary tubediameter, capillary tube length, and pump specifications. The databasemay be populated with data from large numbers of previous cleaning runs,but preferably includes at least the pressure and flow rate measurementsof a new (entirely clean) capillary tube of the same diameter,substantially the same length (for example, within 10 feet), and theother parameters being substantially the same. A numerical scale can beestablished with the flow rate through the new capillary tube beingconsidered, for example, 100% flow (the highest cleanliness rating) anda hypothetical plugged tube being considered, for example, 0% flow (thelowest cleanliness rating). The numerical scale may or may not be linearand the end points of the scale may differ from this example.

Optionally, the database will include a second record that includes thepressure and flow rate measurements of a heavily restricted orsubstantially plugged (but not completely plugged) capillary tube withall other conditions or variables being substantially identical to theconditions or variables under which the measurements of the newcapillary tube were made. If the two records for the new and heavilyrestricted capillary tubes were made at the same pressure (such as usingthe same constant pressure pump), then the respective flow rates mightestablish end points of a broad range of flow rates (or scale ofcleanliness) that are possible. Other types of scales are similarlypossible. For example, if the records for each capillary tube cleaningrun include the initial flow rate and the total volume of solidsremoved, then it is possible to calculate an effective volume for acapillary tube and a function of flow rate. Accordingly, an effectivevolume scale could be adopted in which a new capillary tube represents100% of the effective volume and a hypothetical plugged capillary tuberepresents 0% effective volume.

The amount of data necessary to establish a useful database can bereduced by standardizing the parameters that are controlled by thecleaning system. For example, a heating system (or even moderate ambientconditions) may be used to assure substantially the same temperaturesare used from one cleaning run to another, such that temperature iseliminated as a variable. Using the same or identical pump for each runmay eliminate another variable, and yet another variable is eliminatedif the pump is a constant pressure or constant flow rate pump. Using thesame cleaning solution or a cleaning solution having substantiallysimilar flow characteristics eliminates a further variable. Accordingly,the database should include at least one new capillary tube record andat least one used capillary tube record for each combination ofcapillary tube diameter and capillary tube length that are likely to beencountered. Only a few nominal diameters of tubing are expected to beused as capillary tubes (i.e., ⅛ and ¼ inch) and capillary tube lengthcan be varied in increments, such as every 10 feet or every 50 feet.

In another optional embodiment, particle size analysis (PSA) isperformed on the solids removed from the capillary tube. It is believedthat particle size will show a correlation with cleanliness of thecapillary tube. For example, if there is a decline in the size of solidparticles being removed from the capillary tube as the flow rateincreases, then particle size measurements may serve as a beneficialprimary or secondary indicator of cleanliness.

The capillary tube cleaning methods may be performed in closed or openloop systems. Methods that involve cleaning the capillary tube in situ(without removal from the well) are one example of an open loop system.In the latter methods, the cleaning solution is not reused and thepressure and flow rate of the cleaning solution must be measured at theinlet (up hole) end of the tube. It should be noted that the up holepressure measurement of the cleaning solution is a back pressure and nota differential pressure. However, if a constant pressure pump is used inthis application, then the primary measurement is a flow ratemeasurement which should be the same at both ends of the tube.

In an optional embodiment, the cleanliness of an in situ capillary tubemay be preliminarily assessed using pressure and flow rate measurementsof the treatment fluids, rather than a cleaning solution. Accordingly, acleaning solution does not need to be introduced into the capillary tubeunless the preliminary assessment determines that the tube containsrestrictions that require removal.

FIG. 2 is a schematic diagram of a system 30 for cleaning a capillarytube coil. A spool 12 securing a capillary tube coil is positioned forcoupling the opposing ends 22, 24 of the tube in fluid communicationwith the system 30. Valves 32 are disposed on the upstream anddownstream ends of the tube to facilitate isolation of the system whilecapillary tube coils are being exchanged. A set of tanks 34, 36, 38 andassociated pumps 35, 37, 39 are provided to selectively supply a fluidto the capillary tube being cleaned. These tanks will generally includea cleaning solution tank, an isopropyl alcohol tank, and a water tank.

Water is not used in the cleaning process but is used in combinationwith an IA plug to displace the cleaning solvent out the tubing. Thecleaning solution is acidic and its removal is necessary for the safetyof those handling the string after cleaning. Also the water could beused for flow rate and pressure drop measurements. These fluids may bepumped into a drain 40 or passed through a filter 42 to a high pressurepump 44 that is capable of flowing fluid through a heavily restrictedcapillary tube. The pipe section on the outlet side of the pump 44includes a set of pressure relief valves 46, a pressure gauge or sensor48 and a temperature gauge or sensor 50.

In the embodiment shown in FIG. 2, the spool 12 securing the capillarytube is secured in a heated chamber 52 that also includes a mechanismfor automatically rotating the spool about one or more axis 54. Heatingand/or rotation may be used to help loosen and remove solids from thewalls of the capillary tube.

The pressure and temperature of fluid exiting the capillary tube (to theright of the chamber 52) is measured by a pressure gauge or sensor 56and a temperature gauge or sensor 58. The fluid may then be optionallypassed through one or more filtration unit 60, a particle size analyzer62, and/or a flow meter 64 before being directed either to a drain 66 orback into a tank containing the same fluid. Accordingly, the system 30can recycle any of the fluid used in the process, but mixtures ofdifferent fluids may be directed to the drain or waste container.

The measurements made by the components of the system 30 may be made andrecorded manually, but it is preferable to make the measurements withelectronic sensors that communicate with a process controller (notshown). The process controller may be a specialized or multipurposecomputer having memory for storing the measurements in a database.Furthermore, the process controller may run an application program thatincludes computer executable instructions for controlling the system 30in a manner consistent with the cleaning methods of the presentinvention. Accordingly, the process controller may control the operationof valves, pumps, heated chambers and rotation mechanisms shown in FIG.2 and receive measurements and other input from the sensors and othercomponents shown in FIG. 2 in order to execute any or all of thecleaning methods of the present invention. Furthermore, the applicationprogram should accept user input, such as through the use of a computerkeyboard, mouse, or touch screen, regarding the capillary tube length,capillary tube diameter, and any user setpoints regarding the degree ofcleanliness required before the tube is determined to be clean.Optionally, the user may instruct the application program to determinecleanliness on the basis of flow rate, total solid volume removed, orthe like by selecting from one or more scales.

The terms “comprising,” “including,” and “having,” as used in the claimsand specification herein, shall be considered as indicating an opengroup that may include other elements not specified. The terms “a,”“an,” and the singular forms of words shall be taken to include theplural form of the same words, such that the terms mean that one or moreof something is provided. The term “one” or “single” may be used toindicate that one and only one of something is intended. Similarly,other specific integer values, such as “two,” may be used when aspecific number of things is intended. The terms “preferably,”“preferred,” “prefer,” “optionally,” “may,” and similar terms are usedto indicate that an item, condition or step being referred to is anoptional (not required) feature of the invention.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A method for removing deposits formed within a capillary tube thathas been used to deliver treatment chemicals into a well, comprising:pumping a cleaning solution through the capillary tube coil, wherein thecleaning solution comprises at least 20 weight percent of a surfactantor dispersant, at least 5 weight percent of a coupling agent, and atleast 10 weight percent solvent.
 2. The method of claim 1, wherein thesurfactant or dispersant is selected from the group consisting of analkyl aryl sulphonate and a phosphate ester.
 3. The method of claim 1,wherein the surfactant or dispersant is a sulphonic acid.
 4. The methodof claim 3, wherein the sulphonic acid is dodecylbenzenesulphonic acid.5. The method of claim 4, wherein the cleaning solution comprisesbetween 30 and 70 percent of a surfactant.
 6. The method of claim 4,wherein the coupling agent is a non-ionic coupling agent such asethylene glycol monobutyl ether.
 7. The method of claim 4, wherein thecoupling agent is a glycol ether.
 8. The method of claim 1, wherein thesolvent is toluene.
 9. The method of claim 1, further comprising:measuring the pressure drop and flow rate of the cleaning solutionpumped through the capillary tube coil; and continuing to pump thecleaning solution through the capillary tube coil until the capillarytube coil is determined to be clean.
 10. The method of claim 9, whereinthe capillary tube coil is determined to be clean if the measuredpressure drop and flow rate are within 15 percent of a pressure drop andflow rate measured on a new capillary tube coil having the same nominallength and diameter under the same nominal pressure and otherconditions.
 11. The method of claim 1, further comprising: pumping thecleaning solution into the capillary tube string at a constant supplypressure; measuring a flow rate of the cleaning solution being pumpedthrough the capillary tube coil; and continuing to pump the cleaningsolution through the capillary tube coil until the measured flow rate iswithin 15 percent of a flow rate that would be expected through a newcapillary tube coil having the same nominal length and diameter underthe same nominal pressure and other conditions.
 12. The method of claim1, further comprising: pumping the cleaning solution through thecapillary tube coil at a constant flow rate; measuring the pressure dropof the cleaning solution between the ends of the capillary tube coil;and continuing to pump the cleaning solution through the capillary tubecoil until the measured pressure drop is within 15 percent of a pressuredrop that would be expected through a new capillary tube coil having thesame nominal length and diameter under the same pressure and otherconditions.
 13. The method of claim 1, further comprising: reversing theflow direction of cleaning solution being pumped through the capillarytube coil.
 14. The method of claim 1, further comprising: allowing thecleaning solution to remain in the capillary tube coil for a periodgreater than one hour before resuming the pumping.
 15. The method ofclaim 1, further comprising: changing the orientation of the capillarytube coil as the cleaning solution is pumped through the capillary tubecoil.
 16. The method of claim 1, further comprising: heating thecapillary tube coil or the cleaning solution so that the capillary tubecoil or the cleaning solution have a temperature greater than ambienttemperature as the cleaning solution is pumped through the capillarytube coil.
 17. The method of claim 1, wherein the capillary tube has adiameter between one-eighth inch and two inches.
 18. The method of claim1, wherein the step of removing water from the tube includes passing analcohol through the tube.
 19. The method of claim 18, wherein thealcohol is isopropyl alcohol.
 20. A method for removing deposits formedwithin a capillary tube that has been used to deliver treatmentchemicals into a well, comprising: flowing a cleaning solution throughthe capillary tube and into the well, wherein the cleaning solutioncomprises a sulphonic acid.
 21. The method of claim 20, furthercomprising: producing hydrocarbons from the well while the cleaningsolution flows through the capillary tube.